How to adjust Tram 1480 (2m/70cm) antenna for MURS

[KC1LML]

Recently I got into MURS radios and am wondering if I can adjust a spare Tram 1480 dual band (2m/70cm) magnetic mount antenna that I no longer use with my VHF/UHF radio. It has set screws for adjusting its length.


I'm no antenna expert, but I can do simple frequency to wavelength calculations. A 1/4 wave antenna for 146MHz (midpoint of 2m band) should be 20.21" long, and a 1/4 wave for 152.5MHz (midpoint of MURS band) should be 19.349" long.

It looks like I need to shorten the whip by .861". I can't adjust it that much. I'll have to cut it off and ruin it for 2m/70cm. That's why I'm asking if anyone has gotten into this sort of thing?

 

[kb5udf]

check swr before u cut. If it's a vhf quarter wave it might be fine on M u r s already

 

[KC1LML]

Good point. I hadn't connected my swr meter till now because I heard a very noticeable magnetic hum on my car roof when I keyed the mic. I assumed swr was way off. As it turns out, it wasn't that bad. About 1.1 at 151.82 MHz and abt .6 at 154.60 MHz. I only get the hum on the 2 lowest channels.

So then I tried my Nagoya UT-308UV, which I want to keep using on 2m/70cm, and it is a little better match. Abt .6 or .7 at the low end and abt .2 at the high end. It only hums a little on the lowest channel.

Funny, I've never heard or noticed any roof hum on 2m/70cm using either of those 2 antennas. I suppose the hum means some power isn't getting out the antenna.

I have a feeling I could eliminate the hum if I shortened the antenna for a little better match.

 

[kb5udf]

Baring other problems that an analyzer could reveal, those SWR readings are about as good as one could hope for.

Regarding the hum, is this on the received signal; that is are you monitoring your own transmission? If so check your battery; alot of HT's make weird noises on t/x with battery issues.

I re-read your post. I have never heard of a hum on the roof from a 50 watt mobile, so I can't help you there.

 

[n1das]

Are you sure the "hum" you describe is not coming out of the car's audio system due to RF pick-up by the electronics? I don't think tuning the antenna any better will help. Also, how much power are you transmitting with? MURS is allowed a maximum of 2W transmit power.

 

[KC1LML]

I have a pair of BTECH MURS-V1 radios and haven't modified them in any way except trying external antennas. I have been testing them on their high setting which is 2 watts.

As far as the hum, it's hot and I was trying to work in the shade of the overhang. Backed the car all the way out and no more hum. Silly rookie mistake!

I did cut .87" off the Tram whip. Now it's hard to see the swr needle moving at all, at either end of the MURS band. I'll find out this weekend if that actually helps performance.